The invention is in the field of metal-coated plastic articles, and sputtering processes for manufacturing such products.
There are many applications for plastic parts which have the appearance of bright metal. One particular example, is the automotive field where the need for greater fuel economy requires the replacement of metal parts with lighter weight plastic. Metal parts having a chromium coating are a tradition in the automotive industry because of their bright appearance and resistance to abrasion, corrosion and other deleterious environmental effects. Several methods for depositing chromium coatings onto plastic have been developed. These methods have various shortcomings, and most of the resulting articles are unsatisfactory for use as exterior trim on an automobile because they are unable to withstand continual exposure to water and sunlight for a useful lifetime.
In a paper entitled "Sputter Deposition Onto Plastics", in the Proceedings of the 18th-Annual Conference of the Society of Vacuum Coaters (Key Biscayne, Fla., Apr. 7-9, 1975), pp. 8-26, John A. Thornton describes a method of depositing 50 to 100 nm thick metal coatings on lacquer covered pieces of ABS plastic by sputtering a chromium target. This paper describes several advantages obtained by magnetically confining the glow discharge plasma. Among these advantages are high deposition rate at low sputtering gas pressure, minimal substrate heating, and excellent adhesion of the coating to the substrate.
The above mentioned paper by Thornton describes a sputtering source in which the glow discharge plasma is confined adjacent a cylindrical post target. In a paper entitled "The Planar Magnetron", Research/Development Vol. 25, No. 1 (January 1974), John S. Chapin describes a sputtering source in which the glow discharge plasma is confined to a closed loop adjacent a substantially planar target. Such confinement is particularly advantageous because a single substrate placed near the target can receive a large fraction of the sputtered material, and the target can be easily elongated to accomodate large substrates.
Sputter-deposition of chromium coatings onto plastic substrates presents several difficult problems. First, the high melting point and brittleness of chromium make it difficult to fabricate targets. Second, the reflectance of the coating is a function of sputtering pressure and other process parameters. For example, the above-mentioned paper by Thornton indicates that the reflectance of chromium coatings on plastic decreased from about 62% at a sputtering gas pressure of 1 millitorr (mT) to about 40% at 4 mT. Third, cracking of the metal coating is a major problem. The incidence of cracking extends from a single crack observable only with a microscope to an extensive pattern easily observable with the unaided eye.
The cause of the cracking of the coating is believed to be stresses both within the coating and arising because of differential thermal expansion between the coating and the substrate. Metal coatings on plastic substrates are particularly susceptible to cracking because most organic polymers have relatively high thermal expansion coefficients compared with metals. Cracking is particularly likely with chromium coatings because chromium has relatively low ductility at room temperature compared with many other metals.
It is known that small amounts, less than 1% by weight, of interstitial impurities, notably nitrogen, cause embrittlement of high purity chromium. In a paper, "An Appraisal of Possible Scavenger Elements for Chromium and Chromium Alloys", J. Less-Common Metals, Vol. 6, pp. 21-25 (1964), N. E. Ryan discloses that the presence during melting and casting of a few weight per cent of scavenger elements removes and stabilizes non-metallic interstitial impurities, such as N, O, C and S. The resulting chromium alloy is more ductile at room temperature. The choice of a particular scavenger element depends upon the particular interstitial impurity which is present. For example, Ta reacts with C, Ce and La react with N and O, and Ti and Zr react with N, O and C.